A typical non-volatile memory device such as a flash memory has seen widespread use by application to multimedia cards because of its capability to retain memory without power supply. In order to meet the recent growing demand for larger memory capacity, further densification of memory cells is required. A general technical problem encountered in achieving densification is the “increase in Yupin effect”. The technical problem involves erroneous writing caused by narrowed spacing between the neighboring elements at the expense of densification More specifically, the narrowed spacing of the neighboring elements increases the parasitic capacitance therebetween and causes unintended writing of a memory cell affected by writing of neighboring memory cell.
A transistor that configures a memory cell of a flash memory is provided with a floating gate and a control gate which generally employ polycrystalline silicon. Also, in case the memory cell manufacturing steps proceed in the bit-line direction and thereafter the word-line direction, an inter-poly insulating film serving as an inter-gate insulating film interposing the floating gate and the control gate has been conventionally formed entirely over the surfaces of the floating gate and the element isolation region in a single film forming step after the floating gate has been processed.
In order to reduce electricity required in writing or erasing the floating gate, increased coupling ratio is required; the coupling ratio being the ratio between the capacitance between the floating gate and the control gate, and the capacitance between the floating gate and the silicon substrate. Thus, it is desirable to increase the dielectric constant of the inter-poly insulating film. One of such techniques, for instance, can be found in “The Effect of Remote Plasma Nitridation on the Integrity of the Ultrathin Gate Dielectric Films in 0.13 μm CMOS Technology and Beyond”, IEEE ELECTRON DEVICE LETTERS, VOL. 22, No. 7, July 2001, p. 327-329 by S. F. Ting and 11 other authors” in which dielectric constant is increased by converting the oxide film into a nitride film.
However, in case a method is employed in which an insulating film overlying the floating gate and an insulating film overlying the element isolation region are formed in a single step as described earlier, dielectric constant of inter-poly insulating film as well as dielectric constant of the insulating film overlying the STI are increased, consequently increasing the parasitic capacitance between the neighboring cells.
To avoid such problem, insulating film of low dielectric constant needs to be formed over the element isolation region whereas insulating film of high dielectric constant needs to be formed between the floating gate and the control gate. However, selective formation of insulating films of varying dielectric constant is likely to require increased number and complexity of steps, which calls for a specific method to meet such objectives in a simple fashion.